Technical Abstract:
The Louisiana sugarcane industry is dependent on the use of glyphosate ripener applications to increase sucrose levels. Initially these applications began in late-August and were limited to the second-ratoon crop harvested at the start of the growing season. Currently, applications have been extended to include all ratoon crops and harvests through November and early December. There have been a number of new varieties released in Louisiana. Unfortunately, the response of many of these varieties to the standard rate (0.188 lb ae/A) of glyphosate has been inconsistent and higher rates are being applied to elicit a response. Glyphosate, formulated with surfactant, was applied at 0.188 lb /A in the first week of August, September, October, and November of 2006 and 2007 to a first-ratoon crop of the cultivar ‘HoCP 96-540’. Sugarcane was hand-harvested at 4, 5, 6, and 7 weeks after treatment (WAT) and theoretical sugar recovery (TRS) was determined. Increases in TRS were obtained with all application timings and sampling intervals except the November application date. Increases of 5 to 16% in TRS occurred in all sections of the stalk, but the greatest increase (29 to 94%) occurred in the top two-thirds of the stalk where sugar accumulation was still occurring at the time of the glyphosate application. In a second study, application of glyphosate at 0.25 and 0.312 lb/A in September 2007 to a plant-cane crop of the cultivar ‘L 97-128’ resulted in a 33-38% increase in TRS over the non-treated check but did not increase TRS levels over the standard 0.188 lb/A rate. At 6 WAT, the increase in TRS resulted in a 19 to 34% increase in sugar yields for all rates based on estimated yields calculated from stalk weights and populations. However, at 8 WAT the percent increase in TRS was less, 9 to10%, and there was no significant increase in per acre sugar yields as cane yields were further reduced. Use of the 0.312 lb/A rate of glyphosate resulted in increased bleaching of the leaves, stunting and reduction in spring shoot numbers in the subsequent first-ratoon crop which translated into a 14% reduction in first-ratoon sugar yields. To take full advantage of glyphosate as a ripener for enhancing sugar levels at harvest and reducing the impact of the application on the subsequent ratoon crop, growers should not apply glyphosate beyond mid-October. Further, they should avoid using higher rates in the hopes of eliciting a ripening response in varieties that don’t respond to the standard rate.